Within recent years considerable attention has been given to the effects of electromagnetic and particle irradiation on the performance of various electronic components and systems. Such interest has been generated by, for example, the need to predict the performance of control and communications systems employed in earth satellites, space probes and ballistic missiles. Those concerned with the development of sophisticated timing and frequency control oscillators have continually searched for methods and apparatus that will reduce or eliminate the sensitivity of these devices to neutron radiation.
Generally, fast neutron (i.e. neutrons having energies in excess of 0.1 MeV) causes a change in a quartz oscillator's resonant frequency. The neutron sensitivity coefficient is the frequency change per neutron fluence. For AT-cut and SC-cut resonators the sign of the neutron sensitivity coefficient is positive. Frequency increases approximately linearly with fluence. For AT-and SC-cut resonators the slopes range from +0.7.times.10.sup.-21 /n/cm.sup.2 at very high fluences (10.sup.17 to 10.sup.18 n/cm.sup.2) to 8.times.10.sup.-21 /n/cm.sup.2 at 10.sup.10 to 10.sup.12 n/cm.sup.2. Sensitivity probably depends somewhat on the quartz defect density and on the neutron distribution. (Thermonuclear neutrons cause more damage than reactors').
A negative neutron sensitivity coefficient has been observed in BT-cut quartz, as mentioned in: Johnson et al., "The Pile Irradiation of Quartz Crystal Oscillators" Phil. Mag 45, 651 (1954). Positive neutron sensitivity coefficients are discussed in J. C. King et al., "Effects of Reactor Irradiation on Thickness Shear Crystal Resonators", Proc. 16th Ann. Frequently Control Symposium p. 8 (1962).
In co-pending application Ser. No. 719,922, entitled "Method of Making Radiation Hardened Crystal Quartz Oscillators", two oscillators, one of which possesses a negative neutron induced frequency shift coefficient and the other a positive frequency shift coefficient are used to compensate for neutron induced frequency shifts. The difference frequency produced by the neutron radiation is used as an error signal that is fed back to compensate the frequency of one of the resonators.
An example of circuitry useful in reducing acceleration induced frequency shifts in crystal oscillators is provided by U.S. Pat. No. 4,575,690 entitled "Acceleration Insensitive Oscillator", issued to Walls and the present inventor.